A single crystal is a piece of material which forms one crystal and has a substantially homogeneous atomic sequence throughout the whole piece.
Processes for preparing single crystals can be roughly classified into two groups, namely a solution process and a melt process. The single crystal of a salt is usually prepared by the solution process. However, the solution process is greatly influenced by impurities and/or different kinds of ions while the melt process suffers less from such drawbacks.
As the melt process, there are known several processes including the Kyropoulos process, the Stober-Stockbarger process, the Verneuli process and the Czochralski process.
The Czochralski process is now briefly explained by making reference to FIG. 1. A surface of a melted raw material 1 is covered with melted B.sub.2 O.sub.3 2. A seed crystal 3 contacts the surface of the melted material 1 and pulls up a single crystal 4 with rotation. In FIG. 1, numerals 5 and 6 denote a crucible and a heater respectively.
In order to maintain a stoichiometric composition of a semiconducting compound containing a volatile component, namely an atom having a high vapor pressure (eg. GaP, GaAs, InP, etc.), the whole system is contained in a high-pressure chamber 7 as shown in FIG. 1, which is pressurized to a pressure higher than 2 Kg/cm.sup.2 with inert gas 8 (eg. argon, nitrogen, etc.). After grown at the high pressure, the single crystal 4 is cooled to room temperature at the same high pressure and then removed from the chamber 7.
During the cooling at the high pressure, convection flow of the high-pressure gas is so vigorous that the single crystal is quickly cooled, which may result in strain in the crystal and a large dislocation density. For example, the dislocation density in the conventional single crystal is as high as 2-5.times.10.sup.4 cm.sup.-2.